Accidentally deleted when migrating a Mailbox from 2003 to 2010

Ok,
When moving a Legacy SBS 2003 Exchange 2003 Mailbox to a User Mailbox on Exchange 2010, a mailbox somehow got deleted (is missing).
Is there a way I can recover the mailbox?
It appears that the domain user account associated with the mailbox also got deleted (why would deleting a mailbox delete a domain user account, anywyay?).
NOTE: I do not see the domain user account on either the old SBS 2003 ADUC or the 2008 Server ADUC.

Anyway, I read that the default Retention Interval for mailboxes in Exchange 2010 is 30 days.
(I do not know how to confirm that this actually is the Retention Interval, so if you know, please tell me.)

I checked the "Disconected Mailboxes" in Exchange 2010, and did not see anyting at all. The filter is set so that I should see the mailbox that was deleted.

Can I recover the mailbox, and if so, what is the easiest way to do this?

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